[Graham]

Any chance either of the Pendle Mabuchi 20k motors would be allowed?

Not too powerful, but more available than any of the others on the list...

I will think about that, the reason I choose 18K was that they are more likely to be closer match to each other than  the higher RPM motors which tend to have a higher torque variation. And they are close to that of the standard RTR car motors, so competition between standard RTR cars and those with alternative equivalent RPM motors should be closer (at least that's my theory). Some techno expert is bound to come on here now and contradict me 
On the otherhand  in all honesty from my experience of running these proxy's the RPM does not really contribute that much in terms of a successful running car, it only plays a minor role in the overall performance of the running car unless the RPM is wildly different.

So the standard Scalextric 18k motor has around100 gm_cm Torque
Predator S-Can 18k rpm is 120gm_cm
Predator slimline 18k rpm is 90gm_cm
Pioneer S-Can 18k is 160gm_cm
H&W S-Can 18k motor is 140gm_cm

But if you go to a Scaleauto Tech1 20K rpm motor its 216gm_ cm